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A quick estimation of how many cycles are missing given how the undersampling affects the ampersands in the clip makes me think that H.samplerate needs to be somewhere around 900. I could be wrong though.
Edit: Doing another pixel count it seems there are 3 samples for every 4 pixels, which would put the correct H.samplerate at 984, which given the horizontal frequency of ≈16.26 kHz results in a neat pixel clock of 16 MHz. Give it a try!
Given the above assumption, the duration of a line is 61.5 μs. With a sync pulse 8 μs long and 984 samples per line, H.synclen should be set to 128. H.backporch and H.active can then be adjusted to hopefully center the image and give it as much underscan as desired.
Since there are 91 columns, I guess an active picture of 91 plus 2 columns for underscan would work well, so try setting H.active to 744 after adjusting H.samplerate.